Conventionally, there has been known a power tool which is driven by a motor, in which a rotating speed of a motor is controlled in accordance with a manipulation amount of a rotating speed adjusting switch which is displaced by manipulation by a user (e.g., see Patent Documents 1, 2, and 3). Normally, the small manipulation amount of the rotating speed adjusting switch causes a motor to rotate at a low speed, and the large manipulation amount of the rotating speed adjusting switch causes a motor to rotate at a high speed.
When a working operation is conducted by using such a power tool, it is sometimes desired to perform a certain working operation with the rotating speed of a motor fixed. In Patent Document 1, the manipulation amount of the rotating speed adjusting switch is mechanically regulated at a plurality of positions thereby to control a motor to the rotating speed set for each regulated position. Accordingly, when the rotating speed adjusting switch is manipulated to each regulated position, a motor is rotated with the rotating speed fixed in accordance with the regulated position and thereby the working operation can be conducted.
However, in a structure disclosed in Patent Document 1, there is a problem that since a motor can be rotated only at the rotating speed set for each regulated position, the rotating speed of a motor cannot be minutely set.
Also, when a working operation is conducted with the power tool, the rotating speed of a motor is often adjusted in accordance with the content of a working operation. For example, in the case of a driver, the face connection of a screw is performed in the low speed rotation range, while the normal fastening of a screw is performed in a high speed rotation range. In the case of a grass mower, entanglement of the grass is removed in a low speed rotation range; the grass against the wall is mowed in a medium speed range; and normal mowing of the grass is carried out in a high speed rotation range.
Here, there is a problem in the case where a working operation is conducted with a motor rotated at a low speed. That is, if an amount of changes in the rotating speed of a motor with respect to the manipulation amount of the rotating speed adjusting switch is large, it is difficult to perform a minute working operation.
Therefore, in order to improve manipulation performance when a motor is rotated at a low speed, it is conceivable to reduce the amount of changes in the rotating speed with respect to the manipulation amount as compared with that in cases where a motor is rotated at a high speed. For example, Patent Document 2 discloses the characteristics indicating that the amount of changes in the rotating speed of a motor during a low speed rotation is smaller than that during a high speed rotation.